Function of PHEX mutations p.Glu145* and p.Trp749Arg in families with X-linked hypophosphatemic rickets by the negative regulation mechanism on FGF23 promoter transcription.

X-linked hypophosphatemic rickets (XLH) is characterized by increased circulating fibroblast growth factor 23 (FGF23) concentration caused by PHEX (NM_000444.5) mutations. Renal tubular resorption of phosphate is impaired, resulting in rickets and impaired bone mineralization. By phenotypic-genetic linkage analysis, two PHEX pathogenic mutations were found in two XLH families: c.433 G > T, p.Glu145* in exon 4 and c.2245 T > C, p.Trp749Arg in exon 22. Immunofluorescence showed that the localization of p.Glu145* and p.Trp749Arg mutant and secretory PHEX (secPHEX) changed, with decreased expression. In a HEK293T cell model co-transfected with PHEX, secPHEX, and FGF23, wild-type PHEX, secPHEX, and FGF23 proteins were distributed in the cell membrane or endoplasmic reticulum, while the mutant was located in the nuclear membrane and cytoplasm. qPCR of p.Glu145* revealed decreased PHEX and secPHEX mRNA expression in cells, with no difference in mRNA expression of p.Trp749Arg. Both mutations decreased intracellular PHEX endopeptidase activity. Western blot analysis showed decrease in mutant and secPHEX protein expression and no FGF23 protein expression in single-transfected PHEX and secPHEX cells. In cells co-transfected with FGF23, PHEX and secPHEX mutation promoted FGF23 expression. Dual-luciferase reporter gene was used to detect the effect of PHEX on FGF23 promoter. The dual-luciferase reporter gene showed that after PHEX overexpression, the activity of mutant firefly luciferase was significantly higher than that of wild type. The regulatory mechanism between PHEX and FGF23 is still unclear, but we found that PHEX is a direct transcriptional inhibitor of FGF23 and affects the expression of FGF23. This study verified the pathogenicity of the two variants and revealed the possible regulatory mechanism between PHEX and FGF23.